This invention relates to stable crosslinked dispersions containing microgel particles. More particularly, the invention relates to stable crosslinked dispersions which are prepared by addition polymerization of (a) first and second ethylenically unsaturated monomers each bearing functionality capable of crosslinking with the other and (b) at least one other monoethylenically unsaturated monomer, in the presence of (I) organic liquid which is a solvent for the polymerizable monomers, but a non-solvent for the resultant polymer and (II) a polymeric dispersion stabilizer containing at least two segments of which one segment is solvated by said organic liquid and a second segment which is of a different polarity than said first segment and is relatively insoluble in said organic liquid. Still more particularly, the invention relates to such stable crosslinked dispersions, wherein the dispersion is further stabilized by an additional stabilizer which, except for solvents, consists essentially of butylated melamine formaldehyde resin.
Stable crosslinked dispersions of the type to which the improvement of this invention applies are well known. Such stable crosslinked dispersions containing microgel particles are taught by U.S. Pat. No. 4,147,688 to Makhlouf et al. That patent teaches crosslinked dispersions wherein crosslinked acrylic polymer microgel particles are formed by free radical addition polymerization of alpha, beta ethylenically unsaturated monocarboxylic acids, at least one other copolymerizable monoethylenically unsaturated monomer and a certain percentage of crosslinking monomer, in the presence of a hydrocarbon dispersing liquid (See abstract, examples and claims). Other crosslinked dispersions containing microgel particles are disclosed in the patent application and patents referred to in the Maklouf et al disclosure.
U.S. Pat. No. 4,025,474 to Porter et al discloses a polyester based coating composition which includes the crosslinked dispersions disclosed by Makhlouf et al. U.S. Pat. No. 4,075,141 to Porter et al disclose carboxylic acid amide interpolymer--based coating compositions including the same crosslinked dispersions. U.S. Pat. No. 4,115,472, also to Porter et al, discloses urethane coating compositions also including the crosslinked dispersions of Makhlouf et al. U.S. Pat. No. 4,055,607 to Sullivan et al discloses thermosetting compositions of (a) solution acrylic polymer, (b) at least 0.5% of microgel particles formed by polymerizing hydroxyl bearing monomers with nonhydroxyl bearing monomers in the presence of the stabilizer disclosed by Makhlouf et al, and (c) melamine resin. The microgel dispersion of Sullivan et al thus contains functionality capable of reacting with the melamine crosslinking agent.
The dispersion stabilizer employed in producing the microgel particles of the Makhlouf et al compositions are generally polymeric and contain at least two segments, with one segment being solvated by the dispersing liquid and the second segment being of different polarity than the first segment and relatively insoluble, compared to the first segment, in the dispersing medium. Included among the dispersion stabilizers referred to in the Makhlouf et al patent are polyacrylates and methacrylates, such as poly (lauryl) methacrylate and poly (2-ethylhexylacrylate); diene polymers and copolymers such as polybutadiene and degraded rubbers; aminoplast resins, particularly high naphtha-tolerant compounds such as melamine formaldehyde resins etherified with higher alcohols (e.g alcohols having 4 to 12 carbon atoms); and various copolymers designed to have desired characteristics (see Col. 5, lines 1-27).
Among the numerous dispersion stabilizers which could be employed in compositions of Makhlouf et al and which are suitable for compositions to which the improvement of this invention may be applied, are those taught by U.S. Pat. No. 3,607,821 to Clarke. Clarke teaches a stabilizer for non-aqueous dispersions wherein the stabilizer is chemically reacted with dispersed particles of the dispersion (Col. 1, lines 36-42). Each co-reactant stabilizer molecule forms from 1 to 10 (preferably 1 to 4) covalent links with the dispersed polymer (Col. 1, lines 50-52). The covalent links between the stabilizer and the dispersed polymer are formed by reaction between chemical groups provided by the stabilizer and complementary chemical groups provided by the dispersed polymer or by copolymerization reaction (Col. 1, lines 63-67).
Particularly preferred dispersion stabilizers of Makhlouf et al and the stabilizers employed in compositions of the preferred type to which the improvement of this invention applies are those which are graft copolymers comprising two polymeric segments with one segment being solvated by the dispersing liquid and usually not associated with polymerized particles of the polymerizable ethylenically unsaturated monomers and the second segment being an anchor polymer of different polarity from the first type and relatively non-solvatable by the hydrocarbon solvent and capable of anchoring with the polymerized particles of the ethylenically unsaturated monomer. This anchor polymer segment contains pendent groups capable of copolymerizing with the ethylenically unsaturated monomers of the dispersion (See Col. 5, lines 28-40 of Makhlouf et al).
One of the serious disadvantages of the aforementioned prior art types of crosslinked dispersions containing microgel particles is their lack of stability, both alone and when employed in other compositions. It has been found that the shelf stability of such crosslinked dispersions is not sufficient to allow shipment and storage of these materials for reasonable periods of time before they are employed without the compositions suffering from separation and settling. Furthermore, it has been found that when such compositions are employed in coating compositions as additives, such as a flow control additive, they tend to settle out and separate from the composition, thus creating a viscosity and "kickout" problem. The crosslinked dispersions to which the invention applies are particularly desirable as flow control additives in those compositions in which metallic flake is employed as a pigment in as much as they afford improved metallic pigment control. Of course, when such additives settle and separate from the composition, the advantages insofar as the pigment control features are concerned, are seriously diminished.
It has been found that by further stabilizing the crosslinked dispersions by addition of butylated melamines in accordance with the teachings of the subject invention, the aforementioned problems can be effectively eliminated, thus providing a stable crosslinked dispersion which does not suffer from separation or settling problems whether on the shelf or incorporated into coating compositions such as thermosetting enamels used for automotive applications.
U.S. Pat. No. 4,425,450, issued to Stanley K. Horvath and assigned to Ford Motor Company discloses and claims compositions incorporating the stabilized crosslinked compositions of this invention as a flow control additive. U.S. Pat. No. 4,425,450 discloses and the above Continuation-In-Part application discloses claims paint compositions comprising hydroxy function, film former crosslinked by an appropriate crosslinking agent and containing the improved stable crosslinked dispersions of this invention as a flow control additive preferred embodiments of the compositions claimed are intermediate and high solids paint compositions. The intermediate solids paint compositions comprise a film forming component comprising a hydroxy functional copolymer having a number average molecular weight (M.sub.n) greater than 5,000 and a specified hydroxy functional monomer percentage, a butylated melamine formaldehyde crosslinking agent and a flow control additive comprising the improved stable crosslinked dispersion of this invention. The claimed high solids paint composition comprises a film forming component consisting essentially of a hydroxy functional copolymer having a number average molecular weight (M.sub.n) of between about between about 1,500 and about 5,000, a specified hydroxy functional monomer percentage and a specified amount of alpha-beta olefinically unsaturated carboxylic acid, a crosslinking agent comprising methylated melamine formaldehyde and a flow control additive comprising the improved stable crosslinked dispersion of this invention.